1. Field of The Invention
The present invention relates in general to an improved metalorganic chemical vapor deposition (MOCVD) of ferroelectric thin films such as doped and undoped Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 thin films using safe and stable precursors, e.g. lead tetramethylheptadione, zirconium tetramethylheptadione, and titanium ethoxide.
2. The Prior Art The lead zirconate titanate [Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 or PZT] ceramics are well known materials with the perovskite structure which have useful ferroelectric and electro-optic properties. Recent studies showed that PZT materials offer high permittivity for capacitors, large spontaneous polarization for nonvolatile memory devices, large electromechanical coupling coefficient for surface acoustic wave (SAW) applications, and good optical properties for electro-optic devices.
A variety of techniques have been used for the deposition of ferroelectric thin films. In general, the thin film deposition techniques can be divided into two major categories; i.e., (1) physical vapor deposition (PVD) and (2) chemical processes. Among the PVD techniques, the most common methods used for the deposition of ferroelectric thin films are electron beam evaporation, rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy (MBE), and laser ablation, see Oikawa et al., "Preparation of Pb(Zr,Ti)O.sub.3 thin films by an electron beam evaporation technique," Appl. Phys. Lett., 29(8), 491 (1976), Okada et al., "Some electrical and optical properties of ferroelectric lead-zirconate-lead-titanate thin films," J. Appl. Phys., 48(7), 2905 (1977), Takayama et al., "Preparation of epitaxial Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 thin films and their crystallographic, pyroelectric, and ferroelectric properties," J. Appl. Phys., 65(4), 1666 (1989), Sreenivas et al., "Surface acoustic wave propagation on lead zirconate titanate thin films," Appl. Phys. Lett., 52(9), 709 (1988), and Ramesh et al., "Ferroeletric Pb(Zr.sub.0.2 Ti.sub.0.8)O.sub.3 thin films on epitaxial Y--Ba--Cu--O," Appl. Phys. Lett., 59(27), 3542 (1991). The chemical processes can be further divided into two subgroups; i.e., the chemical vapor deposition (CVD) and the wet chemical process including sol-gel process and metalorganic decomposition (MOD). The first successful PZT films were produced by PVD techniques (e-beam evaporation and rf diode sputtering) in 1976, then followed by MOD process and sol-gel technique in mid 1980s, see Fukushima et al., "Preparation of ferroelectric PZT films by thermal decomposition of organometallic compounds," J. Mater. Sci., 19, 595 (1984) and Yi et al., "Preparation of Pb(Zr,Ti)O.sub.3 thin films by sol-gel processing: electrical, optical, and electro-optic properties," J. Appl. Phys., 64(5), 2717 (1988). The laser ablation and metalorganic chemical vapor deposition (MOCVD) PZT films did not appear until the beginning of the 1990s, see Sakashita et al., "Preparation and electrical properties of MOCVD-deposited PZT thin films," J. Appl. Phys., 69(12), 8352 (1991) and Peng et al., "Low temperature metalorganic chemical vapor deposition of perovskite Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 thin films," Appl. Phys. Lett., 61(1), 16 (1992). The PVD techniques require a high vacuum, usually better than 10.sup.-5 torr, in order to obtain a sufficient flux of atoms or ions capable of depositing onto a substrate. The advantages of the PVD techniques are (1) dry processing, (2) high purity and cleanliness, and (3) compatibility with semiconductor integrated circuit processing. However, these are offset by disadvantages such as (1) low throughput, (2) low deposition rate, (3) difficult stoichiometry control, (4) high temperature post deposition annealing, and (5) high equipment cost. Laser ablation is a newly developed thin film deposition technique and the understanding of this process is in its infant period. Laser ablation has found some success in depositing high temperature superconducting films. There are only a few reported works on laser deposition of PZT films. The major problem of this technique are the composition and thickness nonuniformity of the deposited films over a large scale.
The wet chemical processes includes MOD and sol-gel process. The advantages of the wet chemical process are: (1) molecular homogeneity, (2) high deposition rate and high throughput, (3) excellent composition control, (4) easy introduction of dopants, and (5) low capital cost; deposition can be done in ambient condition, no vacuum processing is needed. The major problems due to this wet process are (1) film cracking during the post-annealing process and (2) possible contamination which results in a difficulty to incorporate this technique into the semiconductor processing. However, because it provides a fast and easy way to produce the complex oxide thin films, this wet chemical process acts a very important role in the investigation of the interrelationship among the processing, the microstructure, and the property of the films.
Of all the above mentioned techniques, the MOCVD technique appears to be the most promising because it offers advantages of simplified apparatus, excellent film uniformity, composition control, high film densities, high deposition rates, excellent step coverage, and amenability to the large scale processing. The excellent film step coverage that can be obtained by MOCVD cannot be equaled by any other technique. Purity, controllability, and precision that have been demonstrated by MOCVD are competitive with the MBE technique. More importantly, novel structures can be grown easily and precisely. MOCVD is capable of producing materials for an entire class of devices which utilize either ultra-thin layers or atomically sharp interfaces. In addition, different compositions, for example Pb(Zr.sub.x Ti.sub.1-x)O.sub.3, can be fabricated using the same sources.
The first successful deposition of oxide-based ferroelectric thin films by CVD was reported by Nakagawa et at. in "Preparation of PbTiO.sub.3 ferroelectric thin film by chemical vapor deposition," Jpn. J. Appl. Phys., 21(10), L655 (1982). They deposited PbTiO.sub.3 films on Pt-coated silicon wafers by using TiCl.sub.4, PbCl.sub.2, O.sub.2, and H.sub.2 O as source materials. Several problems arose from their studies: (1) very high evaporation temperature (700.degree. C.) was required of PbCl.sub.2, (2) poor ferroelectric properties (P.sub.r =0.16 .mu.C/cm.sup.2 and E.sub.c =14.5 kV/cm), (3) poor composition uniformity in the bulk of PbTiO.sub.3 films, and (4) crystallographic imperfections due to water and/or chloride contamination. Obviously, chlorides are not ideal precursors for the CVD process. In general, metalorganic precursors have relatively high vapor pressures at lower temperatures. By carefully selecting the organic compounds, the undesirable contaminations in the films can be completely excluded. Metalorganic compounds are now used almost exclusively for the deposition of oxide thin films. During the past few years, five other research groups sequentially published the results of MOCVD PZT films. However, the reported MOCVD PZT films were fabricated at relatively high temperatures (&gt;600.degree. C.) using cold wall type reactors. Cold-wall reactors are good for epitaxial films because of less particle contamination. However, cold-wall reactors suffer from their low throughput. In contrast, hot-wall reactors are conceptually simpler because of the isothermal environment. In addition, they offer much higher throughput than do cold-wall reactors. But hot-wall reactors suffer from particle contaminations from the walls of the chamber. This may not be a severe problem for polycrystalline films. Furthermore, low temperature deposition processes are needed to integrate ferroelectrics into semiconductor processing.
Greenwald, U.S. Pat. No. 5,104,690 granted Apr. 14, 1992, described the art of manufacturing PZT films by CVD. However, Greenwald failed to demonstrate the composition control, the structure property, and the ferroelectric properties of the resulting films. In addition, Greenwald utilized an extremely toxic precursor material, i.e. tetraethyl lead, for the lead component. Also, Greenwald did not show what deposition temperatures he used to obtain PZT films.